Zinc-electroplated steel sheet has excellent corrosion resistance and therefore is used in various fields. The corrosion resistance of the plated layer thereof is usually enhanced by forming a chromate film on the surface thereof. However, the chromate film is extremely thin and lacks uniformity in thickness. It is easily scratched off and its corrosion resistance enhancement effect is limited.
Therefore in recent years, attempts have been made to improve the corrosion resistance of the plated layer by incorporating therein an element or elements in addition to zinc. Zn-Ni-alloy electroplating of steel sheets is a typical example. However, the conventional Zn-Ni-alloy-electroplated layer contains 8-16 wt % of expensive Ni, and the steel sheet must be plated as thickly as 20 g/m.sup.2 (per side) in coating weight in order to obtain stable corrosion resistance 3-4 times greater than the corrosion resistance of the conventional Zn-electroplated steel sheets. The cost therefor is equivalent to that for providing a conventional Zn-electroplated layer with double thickness. Therefore, known Zn-Ni-alloy-electroplated steel sheets can compete with conventional Zn-electroplated steel sheets in fields where Zn-electroplated steel sheets with coating weight of 40 g/m.sup.2 (per side) or more must be used. But the Zn-Ni-alloy-electroplated steel sheet cannot compete with the conventional Zn-electroplated steel sheet in the fields where Zn-electroplated steel sheet plated as thickly as 40 g/m.sup.2 (per side) does not have to be used, since the Zn-electroplated steel sheet is less expensive.
We made an extensive study in order to develop a Zn-Ni-alloy electroplated steel sheet which is superior to known Zn-Ni-alloy electroplated steel sheets in corrosion resistance with the same coating weight, and we have found that such a Zn-Ni-alloy electroplated steel sheet can be obtained by causing a slight amount of titanium(in the form of some titanium compounds) to deposit in a finely distributed state in the Zn-Ni-alloy plated layer. As a result of a further detailed study, it was found that it is adequate for the Zn-Ni-alloy-plated layer containing 8-16% by weight Ni to contain titanium in the amount of 0.0005-1% by weight as titanium.
The Zn-Ni-alloy-electroplated steel sheet having such a composition exhibits considerably good corrosion resistance with a single plated layer, and can compete with the inexpensive Zn-electroplated steel sheet in fields where high corrosion resistance is not required.
The titanium-compound-containing Zn-Ni-alloy electroplated steel sheet can be obtained by electroplating steel sheet with an acidic, preferably sulfuric acid acidic, electroplating bath containing 10-40 g/l ZN.sup.2+, 15-160 g/l Ni.sup.2+, 0.2-10 g/l Ti.sup.4+, whereby the Ni.sup.2+ /(Zn.sup.2+ +Ni.sup.2+) ratio is adjusted to be about 0.2-0.8 in the molar concentration. (Japanese Laid-Open Patent Publication No. 104194/83).
However, in the case where electroplated steel sheet is used for automobile body exterior panel, which may be hit by stones kicked up by the tires or by other cars, the plated layer may peel off at the spot where the stone hits. When used in such an application, it is preferred that the steel sheet be first pre-electroplated with a Zn-Ni-alloy, and then be plated with the Zn-Ni-alloy containing titanium compounds of the composition as mentioned above as the principal plated layer. The pre-plated layer should be a Zn-Ni-alloy containing 12-87% by weight Ni and have a thickness of 0.05-1 .mu.m.
When a steel sheet is plated with two layers, that is, when the steel sheet is pre-plated with a nickel-rich Zn-Ni-alloy layer, the steel sheet is first pre-plated with a plating bath containing zinc ions Zn.sup.2+ and nickel ions Ni.sup.2+ whereby the Ni.sup.2+ /(Zn.sup.2+ +Ni.sup.2+)ratio is adjusted to be 0.72-0.86 in the molar concentration (0.70-0.85 in the weight ratio) and then is plated with the same plating bath as described above (Japanese Laid-Open Patent Publication No. 85889/84).
When a steel sheet is electroplated with the electroplating bath containing zinc ions, nickel ions and titanium ions as described above, however, the amount of the deposited titanium compounds varies in accordance with the time course change of the bath and fluctuation in the plating conditions, and the corrosion resistance of the plated sheet may vary. It was also confirmed that the deposition of the titanium compounds are stabilized by addition of a small amount of each of one or more of aluminum ions, magnesium ions, ferric ions, indium ions and antimony ions to the bath containing zinc, nickel and titanium. The reason why the deposition of the titanium compound is stabilized by addition of aluminum ions, etc. is not yet fully understood. But it was confirmed that the thus plated layer contains a slight amount of aluminum, iron, chromium, indium or antimony when aluminum ions, ferric ions, chromium ions, indium ions or antimony ions are contained in the plating bath.
There has been known no measure for after-treatment of the thus plated layer practised other than the chemical conversion, such as chromating, when a highly corrosion-resistant plated layer is formed by improvement of the composition of the plated layer. We made a study in search of an after-treatment method which can improve the corrosion resistance of the Zn-Ni-alloy-electroplated layer containing titanium compounds and found that the corrosion resistance of the plated layer is enhanced by heating the plated steel sheet in the presence of water.